Balanced positioning system for use in lithographic apparatus

ABSTRACT

A balanced positioning apparatus comprises a balance mass which is supported so as to be moveable in the three degrees of freedom, such as X and Y translation and rotation about the Z-axis. Drive forces in these degrees of freedom act directly between the positioning body and the balance mass. Reaction forces arising from positioning movements result in corresponding movement of the balance mass and all reaction force are kept within the balanced positioning system. The balance mass may be a rectangular balance frame having the stators of two linear motors forming the uprights of an H-drive mounted on opposite sides. The cross-piece of the H-drive spans the frame and the positioned object is positioned within the central opening of the frame.

This application is a continuation of co-pending U.S. patent applicationSer. No. 10/322,800, filed Dec. 19, 2002, which is a continuation ofU.S. patent application Ser. No. 09/739,097, filed Dec. 19, 2000, nowU.S. Pat. No. 6,525,803, which claims priority from European applicationno. 99310371.2, filed Dec. 21, 1999 and European application No.99310324.1, filed Dec. 21, 1999, the contents of each application isincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to balanced positioning systems, such asmay be used to position a moveable object in at least three degrees offreedom. More particularly, the invention relates to the use of such abalanced positioning system in lithographic projection apparatuscomprising:

-   -   an illumination system for supplying a projection beam of        radiation;    -   a first object table for holding patterning means capable of        patterning the projection beam according to a desired pattern;    -   a second object table for holding a substrate; and    -   a projection system for imaging the patterned beam onto a target        portion of the substrate.

DESCRIPTION OF THE RELATED ART

The term “patterning means” should be broadly interpreted as referringto means that can be used to endow an incoming radiation beam with apatterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate; the term “light valve” hasalso been used in this context. Generally, the said pattern willcorrespond to a particular functional layer in a device being created inthe target portion, such as an integrated circuit or other device (seebelow). Examples of such patterning means include:

A mask held by said first object table. The concept of a mask is wellknown in lithography, and its includes mask types such as binary,alternating phase-shift, and attenuated phase-shift, as well as varioushybrid mask types. Placement of such a mask in the projection beamcauses selective transmission (in the case of a transmissive mask) orreflection (in the case of a reflective mask) of the radiation impingingon the mask, according to the pattern on the mask. The first objecttable ensures that the mask can be held at a desired position in theincoming projection beam, and that it can be moved relative to the beamif so desired.

A programmable mirror array held by a structure, which is referred to asfirst object table. An example of such a device is a matrix-addressablesurface having a viscoelastic control layer and a reflective surface.The basic principle behind such an apparatus is that (for example)addressed areas of the reflective surface reflect incident light asdiffracted light, whereas unaddressed area, reflect incident light asundiffracted light.

Using an appropriate filter, the said undiffracted light can be filteredout of the reflected beam, leaving only the diffracted light behind; inthis manner, the beam becomes patterned according to the addressingpattern of the matrix-addressable surface. The required matrixaddressing can be performed using suitable electronic means. Moreinformation on such mirror arrays can be gleaned, for example, from U.S.Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein byreference.

A programmable LCD array, held by a structure which is referred to asfirst object table. An example of such a construction is given in U.S.Pat. No. 5,229,872, which is incorporated herein by reference.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask;however, the general principles discussed in such instances should beseen in the broader context of the patterning means as hereabove setforth.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The illumination system may also includecomponents operating according to any of these design types fordirecting, shaping or controlling the projection beam of radiation, andsuch components may also be referred to below, collectively orsingularly, as “lens”. In addition, the first and second object tablemay be referred to as the “mask table” and the “substrate table”,respectively.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningmeans may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(comprising one or more dies) on a substrate (silicon wafer) that hasbeen coated with a layer of radiationsensitive material (resist). Ingeneral, a single wafer will contain a whole network of adjacent targetportions that are successively irradiated via the projection system, oneat a time. In current apparatus, employing patterning a mask on a masktable, a distinction can be made between two different types of machine.In one type of lithographic projection apparatus, each target portion isirradiated be exposing the entire mask pattern onto the target portionat once; such an apparatus is commonly referred to as a wafer stepper.In an alternative apparatus—commonly referred to as a step-and-scanapparatus—each target portion is irradiated by progressively scanningthe mask pattern under the projection beam in a given referencedirection (the “scanning” direction) while synchronously scanning thesubstrate table parallel or anti-parallel to this direction; since, ingeneral, the projection system will have a magnification factor M(generally <1), the speed V at which the substrate table is scanned willbe a factor M times that at which the mask table is scanned. Moreinformation with regard to lithographic devices as here described can begleaned, for example, from U.S. Pat. No. 6,046,792, incorporated hereinby reference.

In general, apparatus of this type contained a single first object(mask) table and a single second object (substrate) table. However,machines are becoming available in which there are at least twoindependently movable substrate tables; see, for example, themulti-stage apparatus described in U.S. Pat. No. 5,969,441 and U.S. Ser.No. 09/180,011, filed 27 Feb. 1998 (WO 98/40791), incorporated herein byreference. The basic operating principle behind such a multi-stageapparatus is that, while a first substrate table is underneath theprojection system so as to allow exposure of a first substrate locatedon that table, a second substrate table can run to a loading position,discharge an exposed substrate, pick up a new substrate, perform someinitial metrology steps on the new substrate, and then stand by totransfer this new substrate to the exposure position underneath theprojection system as soon as exposure of the first substrate iscompleted, whence the cycle repeats itself; in this manner, it ispossible to achieve a substantially increased machine through but, whichin turn improves the cost of ownership of the machine.

In a known lithographic apparatus, the drive unit of the positioningdevice for the substrate table comprises two linear Y-motors each ofwhich comprises a stator extending parallel to the Y-direction andsecured to a base of the positioning device, and a translator (Y-slider)movable along the stator. The base is secured to the frame of thelithographic device. The drive unit further comprises a linear X-motorthat comprises a stator extending parallel to the X-direction and atranslator (X-slider) which can be moved Along the stator. The stator ofthe X-motor is mounted on an X-beam that is secured, near its respectiveends, to the translators (Y-sliders) of the linear Y-motors. Thearrangement is therefore H-shaped, with the two Y-motors forming theuprights and the X-motor forming the cross-piece, and this arrangementis often referred to as an H-drive.

The driven object, in this case the substrate table, can be providedwith a so-called air foot. The air foot comprises a gas bearing by meansof which the substrate table is guided so as to be movable over a guidesurface of the base extending at right angles to the Z-direction.

In a lithographic apparatus, reactions on the machine frame toacceleration forces used to position the mask (reticle) and substrate(wafer) to nanometer accuracies are a major cause of vibration,impairing the accuracy of the apparatus. To minimize the effects ofvibrations it is possible to provide an isolated metrology frame, onwhich all position sensing devices are mounted, and to channel allreaction forces to a so-called force or reaction frame that is separatedfrom the remainder of the apparatus.

In an alternative arrangement, the reaction to the driving force ischanneled to a balance mass, which is normally heavier than the drivenmass which is free to move relative to the remainder of the apparatus.The reaction force is spent in accelerating the balance mass and doesnot significantly affect the remainder of the apparatus. Balance massesmoveable in three degrees of freedom in a plane are described in WO98/40791 and WO 98/28665 (mentioned above), as well as U.S. Pat. No.5,815,246.

EP-A-0,557,100 describes a system which relies on actively driving twomasses in opposite directions so that the reaction forces are equal andopposite and so cancel out. The system described operates in twodimensions but the active positioning of the balance mass necessitates asecond positioning system of equal quality and capability to thatdriving the primary object.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a balancing system thatis readily extendable to multiple degrees of freedom and is usable withvarious different drive mechanisms.

According to the present invention there is provided a lithographicprojection apparatus comprising:

-   -   an illumination system for supplying a projection beam of        radiation;    -   a first object table for holding patterning means capable of        patterning a projection beam according to a desired pattern;    -   a second object table for holding a substrate; and    -   a projection system for imaging the patterned beam onto a target        portion of the substrate; and    -   a balanced positioning system capable of positioning at least        one of said object tables in more than three degrees of freedom,        the positioning system comprising:    -   at least one balance mass;    -   bearing means for movably supporting said balance mass;    -   coarse positioning means for positioning said object table in        first to third degrees of freedom, said three degrees of freedom        being translation in first and second directions and rotation        about a third direction, said first, second and third directions        being substantially mutually orthogonal; and    -   fine positioning means for positioning said object table in at        least a fourth degree of freedom substantially orthogonal to        said first, second a third degrees of freedom, said coarse and        fine positioning means being arranged so that reaction forces        from said coarse and fine positioning means are channeled to        said balance mass; characterized in that:    -   said balance mass is supported by said bearing means so as to be        substantially free to move in at least said fourth degree of        freedom.

The long stroke (coarse) positioning system of a lithography apparatusis normally arranged to position the apparatus in X, Y and Rz degrees offreedom whilst a short stroke (fine) positioning system provideshigher-precision positioning over all 6 degrees of freedom (i.e. X, Y,Z, Rz, Ry, and Rx). The positioning movements of the short strokepositioning system can be a source of undesirable vibrations in theapparatus. These movements are often of much higher frequency thanmovements of the long stroke positioning system and can involve highaccelerations so that the reaction forces are large, even though themoving mass is smaller. By arranging for the reaction forces of the finepositioning means to be channeled to the balance mass, which is free tomove in at least one additional degree of freedom, directly or via thecoarse positioning means, the present invention ensures that allreaction forces are confined to the balanced positioning system andvibrations in the remainder of the apparatus are minimized.

The balance mass may be a single body moveable in at least four degreesof freedom or may be made up of several parts separately moveable in oneor more degrees of freedom. For example, in an embodiment of theinvention a first part of the balance mass is a frame moveable in thefirst to third degrees of freedom (e.g. X, Y and R_(z)) and surroundingthe object table whilst a second part of the balance mass is disposedunderneath the object table and is moveable in at least the fourthdegree of freedom (e.g. Z).

According to a further aspect of the present invention there is provideda lithographic projection apparatus comprising:

-   -   an illumination system for supplying a projection beam of        radiation;    -   a first object table for holding patterning means capable of        patterning the projection beam according to a desired pattern;    -   a second object table for holding a substrate; and    -   a projection system for imaging the patterned beam onto a target        portion of the substrate; and    -   a balanced positioning system capable of positioning at least on        of said object tables in at least three degrees of freedom, the        positioning system comprising:    -   at least one balance mass;    -   bearing means for supporting said balance mass so as to be        substantially free to move in said three degrees of freedom; and    -   driving means for acting directly between said object table and        said balance mass to position said object table in said three        degrees of freedom; characterized in that:    -   said balance mass comprises a generally rectangular frame having        its sides generally parallel to said first and second        directions, and a central opening in which said object table is        at least partly disposed.

With the balance mass in the form of a rectangular frame, the drivesforming the uprights of a so-called H-drive arrangement can easily beintegrated into the sides of the frame ensuring that the reaction forcesall act directly between balance mass and driven object table. Also,because the driven object table sits within the central opening of thebalance frame the distance in the Z-direction between the centers ofgravity of the balance frame and the driven mass is reduced.

To reduce the excursions of the balance mass, and hence the overallfootprint of the apparatus, it is preferred that the balance mass isconsiderably more massive, preferably at least five times, than thepositioned object. In this regard, all masses that move with the balancemass are considered part of it and all masses that move with thepositioned object are considered part of that.

It should be noted that in embodiments of the invention according toeither of the aspects described above, multiple object (mask orsubstrate) tables may be provided and the reaction forces to the driveforces of two or more tables may be directed to a common balance mass ormasses.

According to yet a further aspect of the present invention there isprovided a lithographic projection apparatus comprising:

-   -   an illumination system for supplying a projection beam of        radiation;    -   a first object table for holding patterning means capable of        patterning the projection beam according to a desired pattern;    -   a second object table for holding a substrate; and    -   a projection system for imaging the patterned beam onto a target        portion of the substrate; and    -   a balanced positioning system capable of positioning at least on        of said object tables in at least two degrees of freedom, the        positioning system comprising:    -   at least one balance mass;    -   bearing means for movably supporting said balance mass;    -   positioning means for positioning said object table in at least        first and second degrees of freedom, said first to second        degrees of freedom being translations in first and second        directions that are substantially orthogonal, said positioning        means comprising coarse and fine positioning means and being        arranged so that reaction forces from said positioning means are        channeled to said balance mass; characterized in that:    -   said coarse positioning means comprises a planar electric motor        having a translator mounted to said object table and a stator        extending parallel to said first and second directions and        mounted to said balance mass.

The forces exerted by the planar motor will be channeled directly to thebalance mass in the first and the second direction as opposed to anH-drive arrangement. In an H-drive arrangement forces may be channeledindirectly to the balance mass, since the object table is driven by anX-slider over an X-beam in the X-direction and the X-beam and objecttable are driven in the Y-direction by two Y-direction linear motorswith corresponding sliders mounted to both ends of the X-beam. Only thebeams of the Y-linear motors are mounted to the balance mass. Forcesexerted in the X-direction by the X-motor will be channeled indirectlyvia the X-beam and the Y-direction linear motors to the balance mass.When a planar motor is used reaction forces in both the X-direction andthe Y-direction are directly channeled to the balance mass. Further,with the stator (e.g. a magnet array) mounted to the balance mass, themass of the balance mass is desirably increased to reduce its movementrange.

In a vacuum environment it may be advantageous to use the planar motoralso to levitate the object table because it will be difficult to use agas bearing to levitate the object table in a vacuum environment. Theplanar motor may also be used to rotate the object table around a thirddirection being mutually orthogonal to said first and second direction.

The magnate levitation of the planar motor provides for a frictionlessbearing allowing the balance mass to, freely move in first and seconddirections and rotate a found the third direction. The balance mass mayalso be movable in the third direction and/or rotatable around one orboth of the first and second directions such that it provides balancingin more than three degrees of freedom. For this purpose the balance massmay be supported by supports having a low stiffness in the thirddirection. The balance mass may be provided with upstanding walls toraise the center of gravity of the balance mass to the same level in thethird direction as the center of gravity of the object table.

According to a further aspect of the invention there is provided amethod of manufacturing a device using a lithographic projectionapparatus comprising:

-   -   an illumination system for supplying a projection beam of        radiation;    -   a first object table for holding patterning means capable of        patterning the projection beam according to a desired pattern;    -   a second object table for holding a substrate; and    -   a projection system for imaging the patterned beam onto a target        potion of the substrate; the method comprising the steps of:    -   providing a substrate provided with a radiation-sensitive layer        to said second object table;    -   providing a projection beam of radiation using an illumination        system;    -   using patterning means to endow the projection beam with a        pattern its cross-section;    -   projecting the patterned beam of radiation onto target portions        of said substrate;    -   wherein during or prior to said projecting step at least one of        said object tables is moved in first to third degrees of freedom        by coarse positioning means and in at least a fourth degree of        freedom by fine positioning means and, during such movement,        reaction forces in said first to third degrees of freedom are        exerted on a balance mass;    -   characterized by the further step of:    -   channeling reaction forces in said fourth degree of freedom to        said balance mass.

In a manufacturing process using a lithographic projection apparatusaccording to the invention a pattern (e.g. in a mask) is imaged onto asubstrate which is at least partially covered by a layer ofradiation-sensitive material (resist). Prior to this imaging step, thesubstrate may undergo various procedures, such as priming, resistcoating and a soft bake. After exposure, the substrate may be subjectedto other procedures, such as a post-exposure bake (PEB), development, ahard bake and measurement/inspection of the imaged features. This arrayof procedures is used as a basis to pattern an individual layer of adevice, e.g. an IC. Such a patterned layer may then undergo variousprocesses such as etching, ion-implantation (doping), metallization,oxidation, chemo-mechanical polishing, etc., all intended to finish offan individual layer. If several layers are required, then the wholeprocedure or variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “target area”or “target portion”, respectively.

In the present document, the terms illumination radiation andillumination beam are used to encompass all types of electromagneticradiation or particle flux, including, but not limited to, ultravioletradiation (e.g. at a wavelength of 365 nm, 248 nm, 193 nm, 157nm or126nm), EUV, X-rays, electrons and ions.

The invention is described below with reference to an orthogonalreference system based on X, Y and Z-axes. The Z direction may bereferred to as vertical but this should not, unless the context demands,be taken as implying any necessary orientation of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described below with reference toexemplary embodiments and the accompanying schematic drawings, in which:

FIG. 1 depicts a lithographic projection apparatus according to a firstembodiment of the invention;

FIG. 2 is a plan view of a balance mass of the present invention in thesubstrate stage of the apparatus of FIG. 1;

FIG. 3 is a view similar to FIG. 2 but additionally showing the drivearrangement for the substrate table;

FIG. 4 is a diagram of the servo system of the: balance system of FIG.2;

FIG. 5 is a plan view of the drift control arrangement of a firstvariation of the balance system of the first embodiment;

FIG. 6 is a plan view of the drift control arrangement of a secondvariation of the balance system of the first embodiment;

FIG. 7 is an enlarged side view of a driver of the drift controlarrangement of FIG. 6;

FIG. 8 is a cross-sectional view of the driver along line I-I of FIG. 7;

FIG. 9 is a plan view of the drift control arrangement of a thirdvariation of the balance system of the first embodiment;

FIG. 10 is an enlarged plan view of a driver of the drift controlarrangement of FIG. 9;

FIG. 11 is a plan view of a fourth variation of the first embodimentshowing stroke limiters;

FIG. 12 is a cross-sectional view of a substrate stage of a secondembodiment of the invention;

FIG. 13 is a cross-sectional view of a substrate stage of a thirdembodiment of the invention; and

FIG. 14 is a cross-sectional view of a substrate stage of a fourthembodiment of the invention.

FIG. 15 is a cross-sectional view of a substrate stage of a fifthembodiment of the invention.

DETAILED DESCRIPTION

In the drawings, like references indicate like parts.

EMBODIMENT 1

FIG. 1 schematically depicts a lithographic projection apparatusaccording to the invention. The apparatus comprises:

-   -   a radiation system LA, IL for supplying a projection beam PB of        radiation (e.g. UV or EUV radiation, x-rays, electrons or ions);    -   a first object table (mask table) MT provided with a mask holder        for holding a mask MA (e.g. a reticle), and connected to first        positioning means for accurately positioning the mask with        respect to item PL;    -   a second object table (substrate table) WT provided with a        substrate holder for holding a substrate W (e.g. a resist-coated        silicon wafer), and connected to second positioning means for        accurately positioning the substrate with respect to item PL;    -   a projection system (“lens”) PL (e.g. a refractive or        catadioptric system, a mirror group or an array of field        deflectors) for imaging an irradiated portion of the mask MA        onto a target portion C of the substrate W.

As here depicted, the apparatus is of a transmissive type (i.e. has atransmissive nask). However, in general, it may also be of a reflectivetype, for example.

The radiation system comprises a source LA (e.g. a Hg lamp, excimerlaser, a discharge plasma source, a laser-produced plasma source, anundulator provided around the path of an electron beam in a storage ringor synchrotron, or an electron or ion beam source) which produces a beamof radiation. This beam is passed along various optical componentscomprised in the illumination system IL,—e.g. beam shaping optics Ex, anintegrator IN and a condenser CO—so that the resultant beam PB has adesired form and intensity distribution.

The beam PB subsequently intercepts the mask MA which is held in a maskholder on a mask table MT. Having passed through the mask MA, the beamPB passes through the lens PL, which focuses the beam PB onto a targetportion C of the substrate W. With the aid of the interferometricdisplacement measuring means IF and the second positioning means, thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the beam PB. Similarly, thefirst positioning means can be use to accurately position the mask MAwith respect to the path of the beam PB, e.g. after mechanical retrievalof the mask MA from a mask library. In general, movement of the objecttiles MT, WT can be realized with the aid of along stroke module (coursepositioning) and a short stroke module (fine positioning), which are notexplicitly depicted in FIG. 1.

The depicted apparatus can be used in two different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected in one go (i.e. a single “flash”) ontoa target portion C. The substrate table WT is then shifted in the Xand/or Y directions so that a different target portion C can beirradiated by the beam PB;

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash”. Instead, themask table MT is movable in a given direction (the so-called “scandirection”, e.g. the Y direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the lens PL (typically,M=¼or ⅕). In this manner, a relatively large target portion C can beexposed, without having to compromise on resolution.

The apparatus also includes a base frame BP (also referred to as a baseplate or machine frame) to support the components of the apparatus, anda reference frame RF, mechanically isolated from the base frame BP tosupport the projection system PL and position sensors such as theinterferometric displacement measuring means IF.

FIG. 2 shows a balance system according to a first embodiment of theinvention, which is used in a wafer stage comprising substrate table WT,of the lithographic apparatus to provide balancing in three degrees offreedom. The arrangement described below may also be used, with suitablemodification, in a reticle stage, comprising; mask table MT, of alithographic apparatus.

The balancing system of the first embodiment comprises a balance frame 2(balance mass) which is supported by substantially frictionless bearings3 so as to be moveable over a guide surface 4 provided on the machinebase frame. The frictionless bearings 3 may be aerostatic bearings orhydrostatic or magnetic bearings, for example. Alternatively, if therequired range of movement is relatively small, elastic guiding systemssuch as flexures or parallel leaf springs can be used. The arrangementmay also be reversed—i.e. the bearings provided in the machine frame andacting against a guide surface on the underside of the balance frame.The guide surface 4 is parallel to the XY plane defined for theapparatus and the balance frame 2 is free to translate in the X and Ydirections and to rotate (Rz) about axes parallel to the Z direction.

The positioning system 10, shown in FIG. 3, is placed within or up thebalancing frame 2 and has relatively large ranges of movement in the Xand Y directions. It is important that the center of mass of thepositioning system 10 be as close as possible in the Z direction to thecenter of mass of the balance frame 2. In particular, it is preferredthat the vertical separation of the two centers of mass is substantiallyless than 100 mm and ideally zero.

Elastic posts or buffers 5 limit the movement of the balance frame 2 toprevent it leaving the guide surface 4.

The positioning system is arranged such that the reaction forces actingin opposition to the drive forces exerted on the driven object aretransmitted to the balance frame via mechanical or electromagneticconnections. These connections are positioned in or close to the XYplane containing the center of mass of the combined system of balanceframe 2 and positioning system 10. The connections may, for example, beaerostatic bearings with bearing surfaces perpendicular to the XY planeor electromagnetic linear actuators with, for example, magnets attachedto the balance frame 2 and coils or armatures attached to thepositioning system, such that the line of action of the electromagneticforces lies in the same XY plane as the combined center of mass.

FIG. 3 shows such an arrangement where the positioning system 10 is aso-called H-drive. The H-drive 10 comprises an X-beam 11 mounted at ornear its ends to respective sliders 12 a, 12 b. Sliders 12 a, 12 b carryarmatures of linear motors that act in concert with elongate magnettracks 13 a, 13 b, which are mounted in the long sides 2 a, 2 b ofrectangular balance frame 2, to translate X-beam 11 in the Y direction.The object to be positioned, in this case wafer table WT, is driven inthe XY plane by a further slider 14 which is positioned on X-beam 11.Slider 14, similarly to sliders 12 a, 12 b, carries the armature of alinear motor to act against a magnet track 15 mounted in X-beam 11 totranslate slider 14 along the X-beam and hence position wafer table WTin the X direction. Independent control of the positions of sliders 12a, 12 b allows the angle between X-beam 11 and the balancing frame to bevaried and hence the Rz (rotation about the Z axis) position of thewafer table WT to be controlled within a certain range to compensate forany yaw movements of the balancing frame. It will be appreciated that,for this reason and also due to distortion of the balance frame causedby shear components in the resultant force on the balance frame, the Xand Y directions in which the drivers exert forces will not always beexactly orthogonal. By this arrangement, the reaction forces in the Yand Rz directions are transferred directly to the balance frame 2.Sliders 12 a, 12 b also carry air bearings 16 a, 16 b which act againstupstanding walls 21 a, 21 b provided on the balance frame 2 to transmitreaction forces in the X direction to balance frame 2. Instead of a pairof thrust bearings 16 a, 16 b to transmit the X direction forces, asingle pre-loaded bearing or an opposed pad bearing, for example, may beused on one of the two sides and is often preferred as it avoidsdifficulties with cosine foreshortening when the X-beam 11 is notperpendicular to the balance frame 2.

As illustrated, the positioning system is supported in the Z directionand against Rx, Ry rotations by the balance frame. This finction canalso be performed b the guide surface 4 for the whole or a part of thepositioning system (e.g. the wafer table WT) by a separate surface orsurfaces fixed relative to the base frame, or by a combination of theabove.

If so-called planar motors are used, reaction forces in the X and Ydirections are transmitted to the balance frame via a magnet (or coil)plate in the XY plane. The magnet (or coil) plate may form part of thebalance frame in the XY plane, and so desirably increase its mass toreduce its movement range. Again, the magnet (or coil) plate may asupported in Z, Rx and Ry directions by a second balance mass or byseparate means, such as frictionless bearings, over the machine base.

The drive forces exerted on the driven object, in this case the wafertable WT, give rise to equal and opposite reaction forces which,according to the invention, are exerted on the balance frame (balancemass). From Newton's laws, it will be seen that the ratio of thedisplacements of the driven object and the balance mass is inverselyproportional to their mass ratio, i.e.: $\begin{matrix}{\frac{X\quad 1\quad(t)}{X\quad 2\quad(t)} = {- \frac{m\quad 2}{m\quad 1}}} & \lbrack 1\rbrack\end{matrix}$

where x₁ is the displacement of mass i relative to the common center ofgravity and m₁ the mass of mass i. In this context it should be notedthat the balance mass ratio may vary according to the direction in whichdisplacement occurs. In the present embodiment, the X-beam 11 andY-sliders 12 a, 12 b move with the wafer table WT for displacements inthe Y direction whereas the wafer table moves relative to the X-beam 11for displacements in the X-direction. Thus the driven mass fordisplacements in the Y-direction is the combined mass of the wafer tableWT, X-slider 14, X-beam 11 and Y-sliders 12 a, 12 b. On the other hand,for displacements in the X-direction, the driven mass is only the massof the wafer table WT and X-slider 14; the X-beam and Y-sliders insteadform part of the balance mass. Since the X-beam and Y-sliders have asimilar mass to the wafer table WT and X-slide 14, this can make asignificant difference to the balance mass ratio.

By making the balance frame 5 to 20 times more massive than the combinedmoving mass of the positioning system, the motion ranges of the balanceframe can be restrained and the overall footprint of the balancingsystem confined as desired.

If, during positioning, the center of mass of the balance frame is notin line with the center of mass of one of the positioning devices in theX or Y direction, reaction forces in that direction may cause yaw motionof the balance frame. In some cases, e.g. circular motion of the drivenobject around a point offset from the center of mass of the balanceframe, yaw motions can be caused that cumulate over time rather thancancel. To prevent excessive yaw motion, a negative feedback servosystem is provided. This control system is also adapted to correctlong-term cumulative translations (drift) of the balance frame thatmight arise from such factors as: cabling to the positioning devices,misalignment of the positioning drives, minute friction in the bearings3, etc. As an alternative to the active drift control system describedbelow, a passive system, e.g. based on low-stiffness springs, may forexample be used.

FIG. 4 shows the control loop of a servo system 30 as referred to above.The X, Y and Rz setpoints of the balance mass with respect to themachine frame are supplied to the positive input of subtractor 31, whoseoutput is passed to the servo controller 32. The servo controllercontrols a three-degree-of-freedom actuator system 33, which applies thenecessary corrections to the balance frame 2. Feedback to the negativeinput of subtractor 31 is provided by one or moremultiple-degree-of-freedom measurement systems 34 which measure theposition of the balance frame and driven mass. The positions of bothbalance frame and driven mass may be measured relative to a fixed frameof reference. Alternatively, the position of one, e.g. the balance mass,may be measured relative to the reference frame and the position of thedriven mass measured relative to the balance mass. In the latter casethe relative position data can be transformed to absolute position dataeither in software or by hardware.

The set points of the servo system 30 are determined so as to ensurethat the combined center of mass of the positioning device(s) andbalance frame 2 remains unchanged in the XY plane. This defines thecondition:M ₁ ·{right arrow over (u ¹ )}( t)+m ₂ ·{right arrow over (u ² )}( t)=m₁ ·{right arrow over (u ¹ )}(0)+m ₂ ·{right arrow over (u ² )}(0)   [2]

where {right arrow over (u_(i))}(t) is the vector displacement of mass iin the X-Y plane at time t relative to a fixed reference point. Theerror signal between the calculated (using equation [2]) and measuredpositions is provided to the actuation system 33, which appliesappropriate correction forces to the balance frame 2. The lowestresonance mode of the balancing frame and/or machine base is at least afactor of five higher than the servo bandwidth of the drift controlsystem.

To minimize cumulative yaw motions of the balancing frame, the controlmode is configured with a low servo bandwidth but a fixed setpoint (e.g.zero yaw). Similar to a passive (e.g. spring) drift control, the servobandwidth for yaw serves as a low-pass filter to minimize transientmoments on the machine base about the yaw axis. In other words, onlyreaction forces to correct long-term (low frequency) movements aretransmitted to the base frame.

FIG. 5 shows a drift control actuation system 33 a according to a firstvariation of the first embodiment. This system comprises three Lorentz(force)-type linear motors (e.g. voice coil motors, ironless multi-phaselinear motors, etc.) 331, 332, 333. Two of these motors 331, 332 act inone direction, e.g. the X direction, and are spaced apart widely in theother, e.g. Y. The third motor 333 acts in the other direction, e.g. Y,and through or near to the combined center of mass of the balancingframe. The drivers are preferably Lorentz force motors having a magnetplate or coil elongate in the direction perpendicular to the directionin which they act so that they can exert a force in the given directionirrespective of the position of the balance frame 2 in the perpendiculardirection.

The above arrangement, using three drivers, is advantageous as being thesimplest possible arrangement, but if the balance frame 2 is an openrectangle with limited resistance to shear, four motors may be used,each acting along or close to the neutral axis of one side member of theframe, thereby to minimize bending of the frame members. Such anarrangement is shown in FIG. 6. Here, four drivers 334 a, 334 b, 334 c,334 d are used—one at each corner, arranged to exert force parallel toand in line with a respective one of the four beams 2 a, 2 b, 2C, 2 d.Each of the four drivers can be, as before, Lorentz type linear motors.A further alternative is to use two planar motors, each exerting forcesin the X and Y directions, to provide combined control in X, Y andR_(z).

An alternative form of driver 334 is shown in FIG. 7, which is a sideview, and FIG. 8, which is a sectional view along line I-I in FIG. 7.Driver 334 consists of a rotary Lorentz motor 335 (such as an ironlessmoving coil motor, a DC or AC brushless motor, etc.) mounted on the baseor machine frame BP and connected to the balance frame 2 by arotary-linear motion transformer 336. The rotary-linear motiontransformer 336 comprises a disc 336 a attached rigidly to the driveshaft 335 a of the motor 335 and having an eccentrically mounted pin 336b. Pin 336 b forms an axle for two wheels 336 c, 336 d which engage acoupling frame 336 e mounted on the balance frame 2. Coupling frame 336e is elongate perpendicular to the direction of action of the force tobe exerted on the balance frame and generally C-shaped in cross-section.It surrounds the wheels 336 c, 336 d so that each engages a respectiveone of opposed bearing surfaces 336 g, 336 f. Bearing surface 336 ffaces towards balance frame 2 and bearing surface 336 g faces away.Thereby, if motor 335 is energized to rotate disc 336 a clockwise inFIG. 8, wheel 336 c will be caused to bear on surface 336 g and exert aleftwards push force on balance frame 2. Similarly anticlockwiserotation of disc 336 a would exert a pull force rightwards on balanceframe 2.

Rotary-linear motion transformer 336 is arranged to be substantiallyfriction free and reversible with no play so that drift actuationcontrol can be carried out in a force mode rather than a position mode.The position of balance frame 2 can additionally be measured via rotaryencoders (not shown) provided on disc 336 a.

A further alternative drift control system is illustrated in FIG. 9,which is a plan view of the balance frame 2, and FIG. 10, which is anenlarged view of one of the drive mechanisms 337 used in thisalternative. Drive mechanism 337 is a so-called “double scara mechanism”which consists of two crank-con'rod mechanisms connected to a commonpivot point. Each crank-con'rod mechanism consists of a crank 337adriven by a Lorentz-type torque motor 337 b and a con'rod 337 cconnecting the end of crank 337 a to common pivot point 337 d. Torquemotor 337 b is mounted on the base frame BP and its drive shaft fixedagainst translation so that reaction forces are transferred to the baseframe BP.

The drift actuation system of FIGS. 9 and 10 is over-determined sinceonly three drives are sufficient to control the balance frame in threedegrees of freedom but the additional motor provides the same benefitsas that of the arrangement of FIG. 6.

The position and orientation of the balance frame 2 can be determinedfrom the crank angles, which may he measured be rotary encoders providedon the drive shaft of motors 337 b. In the servo control system, twocoordinate transforms are provided: one to convert information of theangular position of the cranks 337 a to X, Y, Rz coordinates of theposition of the balance mass 2; and one to convert the forces determinedby the controller 33 into torques for the drive motors 337 b.

As mentioned, the above-described drift control arrangements may includelinear or rotary position sensors incorporated into the linear or rotarydrive mechanisms. Alternatively an independent position measuringsystem, e.g. a grid encoder or a 2-dimensional position sensingdetector, may be employed. Such a system may have multiple outputs whichcan be transformed into X, Y and Rz coordinates or may provideindependent measurement of the XY positions of two points on thebalancing frame, preferably diagonally opposite comers. Such apositioning mechanism may measure the position of the balance frame 2relative to the base frame or, in ultra-precision machines, to avibration-isolated metrology frame.

To prevent the balance frame 2 from drifting out of range, e.g. in theevent of an error situation, a stroke limiting device may be providedbetween the balancing frame and the base frame. An example of such adevice is shown in FIG. 11 which is a view showing a cross-sectionthrough the lower part of the balancing frame 2. In this device, threepins 40 project upwardly from the bearing surface 4 of the base frame BPand engage open-ended slots 41 in the balance frame 2. The slots 41 andpins 40 are sized and arranged to confine movement of the balance frame2 to a predefined envelope in X, Y and Rz. The pins 40 may be resilientor spring-loaded to cushion any shock to the balance frame 2 in theevent of crash. The stroke limiting device may be kinematicallyinverted, with pins projecting from the balance frame 2 engaging inslots on the base frame BP.

If it is not possible to arrange that the centers of mass of thepositioning devices and the balance frame 2, as well as the drivingforces of the various actuators, lie in the same XY plane, drivingforces acting at the offset will cause tilting moments Tx, Ty, i.e.moments tending to rotate the balance frame 2 and the positioningdevices around the X and Y axes. If the balance frame 2 is supported inthe Z, Rx and Ry directions with relatively high stiffness, tiltingmoments Tx, Ty will be transmitted to the base frame BP and causevibrations there. Also, although coarse positioning is usually onlyperformed in the X, Y and Rz directions, fine positioning actuatorsincluded in substrate table WT for the moveable object are commonlycapable of positioning in all six degrees of freedom. The reactionforces from motions of the fine positioning system in Z, Rv, Rx, as wellas the other degrees of freedom can also cause vibrations if transmittedto the base frame BP.

Accordingly, the balance frame 2 is supported in the Z, Rx and Rydirections with low stiffness supports, comprised in bearings 3. Suchsupports may be low-stiffness frictionless bearings or elastic of gassprings in combination with frictionless bearings. Large-gap airbearings may also be used. As with the use of passive components tocontrol drift in X, Y and Rz directions, the spring constants are chosenso that the Eigen-frequency of the balance frame mass-spring system issubstantially lower, e.g. by a factor of 5 to 10, than the lowestfundamental frequency of the motion of the positioning devices. Shouldthe wafer table WT be supported in Z, Rx, Ry by the guiding surface 4 onthe base frame rather than on the balance frame, the base frame memberproviding guiding surface 4 can be treated as a second balance mass forZ, Rx and Ry and be passively supported as described.

EMBODIMENT 2

The substrate stage, comprising substrate table WT, of a secondembodiment of the invention, which may be the same as the firstembodiment save as described below, is shown in FIG. 12.

In the second embodiment, the balance mass 406 takes the form of an openbox with a flat interior base 407, forming a guide surface for the wafertable WT, and standing side walls 408 serving to raise the center ofgravity of the balance mass 406. The substrate table WT includes a finepositioning mechanism 417 operating in 6 degrees of freedom for thesubstrate W and a so-called air-foot forming a substantiallyfrictionless bearing allowing the substrate table WT to be moved overthe guide surface 407.

Movement of the substrate table WT is effected by the coarse positioningmechanism. This includes X-beam 415 relative to which the substratetable WT is driven y an X-driver (not shown) which has at its endssliders 411 which include the translators of Y-direction linear motorsto drive the X-beam, and hence the substrate table WT, in the Ydirection and by applying different forces to the opposite ends of theX-beam in R_(z). The stators 409 of the Y-direction linear motors areprovided in shoulders of the balance mass 406. Y-direction and R_(z)reaction forces from movement of the substrate table are thus directlyapplied to the balance mass 406. X-direction reaction forces aretransferred to the balance mass 406 via bearings between the slides 411and the sidewalls 408 of the balance mass 406.

Because the substrate table WT is guided over the base 407 of thebalance mass 406, Z, R_(y) and R_(x) reaction forces from thecorresponding movements of the substrate WT by the fine positioningmechanism 417, are also transmitted directly to the balance mass 406.Any tilting movements T_(x) T_(y) arising from imperfect adjustment ofthe centers of gravity of the substrate table WT and balance mass 406 aswell as the lines of force exerted by the X- and Y-drives, are alsotransmitted to the balance mass 406 via the air-pot 419 and thestiffness of the Y-linear motors.

To enable the balance mass 406 to absorb the reaction forces in a sixdegrees of freedom it must be free to move in all six degrees offreedom. This is achieved by supporting it from the base frame BP by aplurality of supports 403, which have a low stiffness in the Zdirection, and substantially frictionless bearings 405, which bear onthe lower surface of balance mass 405. The lower surface of balance mass406 is flat, or has flat regions of sufficient size to accommodate themaximum expected or allowed range of movement of balance mass 406. Sincethe balance mass 406 is much, e.g. 5 to 10 times, heavier than thesubstrate table WT, the range of movement of the balance mass 406 willbe much less than the range of movement of the substrate table WT.

EMBODIMENT 3

FIG. 13 depicts the substrate stage, comprising substrate table WT, of athird embodiment of the invention, which may be the same as the first orsecond embodiments described above.

In the third embodiment, the balance mass is divided into two parts 506,507. The first balance mass part 506 comprises a rectangular framesurrounding the subs ate table WT. Opposite sides of the first balancemass part 506 have mounted thereon the stator, e.g. the magnet track, ofthe Y-direction linear motors. The translators, e.g. coils, of theY-direction linear motors, are mounted in sliders 511 at the ends ofX-beam 515. The X-beam includes the stator of X-linear motor and thetranslator is mounted to the substrate table WT. Y- and R_(z)-reactionforces from the Y linear motor, which forms the coarse positioningmechanism together with the X-linear motor, are transmitted directly tothe first balance mass part 506 and the Y-reaction forces aretransmitted via thrust bearings (not shown). To absorb the X- andY-reaction forces, the first balance mass part 506 is supported bysubstantially frictionless bearings, e.g. air bearings, 505 allowing itto move in X, Y and R_(z).

Second balance mass part 507 takes the form of a plate and is disposedunderneath the substrate table WT. Its upper surface 508 is flat andforms a guide surface over which the substrate table WT is borne by airfoot 519. In this way, any reaction forces in Z, R_(x) and R_(y) frommovements of the wafer W by fine positioning mechanism 517 aretransmitted to second balance mass part 507, which is supported from thebase frame BP by a plurality of supports 503 having a low stiffness inthe Z-direction. These supports may be, for example, mechanical or gassprings.

EMBODIMENT 4

A fourth embodiment of the invention is a modification of the secondembodiment for use in a vacuum. The substrate stage, including substratetable WT, is shown in FIG. 14.

As in the second embodiment, the balance 606 takes the form of an openbox. In this embodiment, the base of the box includes the stator 627,e.g. magnet array, of a planar motor whose translator 635 is mounted tothe wafer table WT. More information on a planar motor can be gleanedfrom U.S. Pat. No. 5,886,432, which is incorporated herein by reference.As before, upstanding walls 625 serve to raise the center of gravity ofthe balance mass 606 to the same horizontal plane as that of thesubstrate table WT. The planar motor 627, 635 may be arranged tolevitate as well as translate the substrate table or additional bearingscan be provided. Reaction forces from the X, Y and possibly, R_(z)translations of the planar motor are channeled to the balance mass 606.

EMBODIMENT 5

A fifth embodiment of the invention is a modification of the fourthembodiment as shown in FIG. 15. As in the fourth embodiment thesubstrate table WT is movable in the plane of the stator 627 of theplanar motor (i.e. the X and Y direction). Reaction forces from the X, Yand R_(z) movements of the coarse positioning mechanism (planar motor)are transmitted directly to the balance mass 606. Reaction forces in alldegrees of freedom for the fine position mechanism 617 are transmittedthrough the stiffness of the planar motor or additional bearingsprovided for the substrate table to the balance mass 606. The balancemass is mounted on bearings 605 and low-stiffness supports 603 in thesame way as the second embodiment.

Whilst we have described above specific embodiments of the invention, itwill be appreciated that the invention may be practiced otherwise thandescribed. The description is not intended to limit the invention. Inparticular it will be appreciated that the invention may be used in thereticle or mask stage of a lithographic apparatus and in any other typeof apparatus where fast and accurate positioning of an object in a planeis desirable.

1. A lithographic apparatus, comprising: a first object table configuredto hold a patterning device; a second object table configured to hold asubstrate; a projection system configured to transfer a pattern onto atarget portion of the substrate; and a balanced positioning systemcapable of positioning one or both of the object tables in more thanthree degrees of freedom, the positioning system comprising: a balancemass; a bearing which movably supports the balance mass; a coarsepositioning actuator configured to position the object table in at leasttwo degrees of freedom, the two degrees of freedom including translationin first and second directions, the first and second directions beingsubstantially mutually orthogonal; and a fine positioning actuatorconfigured to position the object table in a least a third degree offreedom substantially orthogonal to the first and second degrees offreedom, the coarse and fine positioning actuators arranged so thatreaction forces from the coarse and fine positioning actuators arechanneled to the balance mass, wherein the balance mass is supported bythe bearing so as be substantially free to move in at least the thirddegree of freedom.
 2. A lithographic apparatus, comprising: a substratetable configured to hold a substrate; a patterning device supportconfigured to hold a patterning device; a projection system configuredto transfer a pattern onto the substrate; a base frame; a balance masssupported by and moveable relative to the base frame and coupled to thesubstrate table, the patterning device support, or both; and asupporting member attached to the balance mass and to the base frame,the supporting member having a stiff portion and at least two pivotpoints.
 3. The apparatus of claim 2, wherein the stiff portion issubstantially inflexible.
 4. The apparatus of claim 1, wherein a portionof the supporting member that includes one of the at least two pivotpoints is rotatable relative to a portion of the supporting member thatincludes another of the at least two pivot points.
 5. The apparatus ofclaim 1, wherein the supporting member comprises at least two base frameconnecting members pivotally attached at one end to the balance mass andat another end to the base frame.
 6. The apparatus of claim 5, wherein avertical position of the balance mass remains substantially constantduring movement.
 7. The apparatus of claim 1, wherein at least one pivotpoint of the supporting member has a pivot axis which is substantiallyperpendicular to a plane in which the balance mass is principallymoveable.
 8. A device manufacturing method, comprising: providing asubstrate that is at least partially covered by a layer ofradiation-sensitive material on a substrate table positioned on a baseframe; projecting a patterned beam of radiation onto the layer ofradiation-sensitive material; moving said substrate table relative tosaid base frame by generating a force between said substrate table and abalance mass; and supporting the balance mass using a supporting membercoupled between the balance mass and the base frame, the supportingmember having a stiff portion and at least two pivot points.
 9. Themethod of claim 8, wherein a portion of the supporting member thatincludes one of the at least two pivot points is rotatable relative to aportion of the supporting member that includes another of the at leasttwo pivot points.
 10. The method of claim 8, wherein the supportingmember comprises at least two base frame connecting members pivotallyattached at one end to the balance mass and at another end to the baseframe.
 11. The method of claim 10, wherein a vertical position of thebalance mass remains substantially constant during movement.
 12. Themethod of claim 8, wherein at least one pivot point of the supportingmember has a pivot axis which is substantially perpendicular to a planein which the balance mass is principally moveable.